Lean burn gasoline and diesel engines are operated at higher than stoichiometric air-to-fuel (A/F) mass ratios for improved fuel economy. Such lean burn engines produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (NOx). The temperature of the exhaust from a warmed up diesel engine is typically in the range of 200° to 400° C. and has a representative composition, by volume, of about 10–17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NOx and the balance nitrogen and water. These NOx gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content and the water content in the hot exhaust stream.
Because the engine-out hydrocarbons in the diesel engine exhaust are not effective in reducing NOx, it has been proposed to add gaseous or vaporizable substances such as ammonia or urea to the oxygen-rich exhaust gas to provide reactants for the conversion of NOx over a suitable catalyst. The practice is termed selective catalytic reduction of NOx (SCR) and catalysts such as certain base metal-exchanged zeolite catalysts have been tested to accomplish this difficult chemical reduction task.
The use of diesel fuel or gasoline constituents as the reductant for NOx is viewed favorably, particularly for vehicle applications, because these fuel component hydrocarbons are stored on-board and a small portion can be diverted for injection into the exhaust gas stream. However, both the engine-out and fuel component hydrocarbons tend to be less reactive and less selective reductants for NOx than ammonia or urea resulting in a higher light-off temperature and undesired hydrocarbon combustion (total oxidation). In hydrocarbon-assisted SCR, ethyl alcohol is viewed as having utility like a hydrocarbon because it can be converted in the exhaust to chemical species useful in the reduction of NOx. Ethyl alcohol can be delivered as a fuel additive, and, if desired, easily distilled off from the fuel and stored in a separate tank on a vehicle.
It is, thus, an object of the present invention to provide an improved method of practicing hydrocarbon-assisted SCR in such oxygen-rich, nitrogen oxide containing mixtures. It is a more specific object of the present invention to provide a two-stage catalytic reactor for successive activation (without total oxidation) of hydrocarbon content in the exhaust and reduction of NOx constituents using different catalysts placed, respectively, in upstream and downstream stages of the reactor.